Barium titanate is a dielectric substance which shows different dielectric properties depending on the crystal phase and particle size of crystals. It is utilized as essential raw material for a multi-layer ceramic capacitor and a positive temperature coefficient resistor (PTCR), or the like. Recently, development of multi-layer ceramic capacitors has a trend of miniaturization and pursuing high capacity. Since the number of layers should increase in order to realize high capacity with a small size, enhancement of powder characteristics of barium titanate such as particle size, shape, purity, crystallinity, or the like is required.
Traditional processes for preparing barium titanate generally include solid processes and liquid processes. According to the solid process, a traditional process for preparing ceramic powder, barium titanate is prepared by essentially reacting a mixture of barium carbonate and titanium oxide with thermal treatment at a high temperature. However, barium titanate powder thus obtained cannot be employed in preparing a ceramic capacitor with a number of layers having high capacity, since it has large particle diameter, broad distribution of particle size, and non-uniform particle shape, and also requires relatively high temperature for sintering. On the other hand, the liquid processes, including hydrothermal synthesis, hydroxide process and sol-gel process, result in most excellent properties in crystallinity of powder, particle size distribution and particle shape. Hydrothermal synthesis is a representative among them, being suitable for preparing a capacitor having a number of layers with high capacity.
As a rule, a hydrothermal synthesis comprises reaction of barium hydroxide with hydrous titanic acid compound. Though barium titanate may be obtained only if the mixture is heated under ambient pressure, reaction under higher temperature and pressure gives barium titanate having high crystallinity. Thus, the reaction utilizes a process with applying high temperature/pressure in an air-tight reactor. Crystalline barium titanate is industrially produced by preparing barium tetrachloride in gel-like hydrate from titanium tetrachloride and barium chloride as the starting material, and mixing barium chloride with an alkaline metal hydroxide as a mineralizer, and reacting the mixture at a temperature between 60° C. and 300° C. under high pressure not less than ambient pressure. According to the process, the feature of particles produced can be controlled by adjusting the parameters such as temperature and pressure. Though calcining stage is required after the synthesis, relatively lower temperature is needed as compared to the solid process. In addition, the process provides barium titanate powder which is suitable for a capacitor having a number of layers with high capacity, since it enables the preparation of barium titanate powder having excellent crystallinity as spherical particles, and small and uniform particle size and distribution.
There are numerous patents regarding such hydrothermal processes. Among them, Korean Patent Publication No. 1993-0002232 discloses a process for preparing ABO3-type perovskite-like ceramic powder, wherein alkaline metal hydroxide as a mineralizer is added to a hydrolyzed aqueous titanium chloride solution and barium chloride as the starting material to carry out hydrothermal reaction, and the residual barium hydroxide in the reactant is treated with an insolubilizer such as carbon dioxide to adjust the molar ratio of Ba/Ti. However, such a process employs excess amount of alkaline metal hydroxide to give residual alkaline metal in the product, to necessitate excess washing process, while expensive anti-corrosive reaction device is needed because chlorine contained in the starting material causes corrosion of the reactor. Further, when the residual barium hydroxide is converted to barium carbonate by treating with an insolubilizer, rod-like barium carbonate having the size of several micrometers is produced, which may not completely participate in calcining stage, to form barium oxide or to be polarized on the surface of barium titanate particle to cause abnormal sintering.
In order to solve those problems described above, Korean Patent No. 10-0428496 and Korean Patent Laid-Open No. 10-2004-0069044 states that a process for preparing particulate barium titanate can be provided, which comprises hydrothermally reacting a mixture of relatively stable titanium hydroxide in an amorphous state without containing chloride ion prepared by sulfuric acid process, and excess amount of barium hydroxide, filtering the reaction mixture, washing with water and drying. However, the reaction is difficult to be completed according to such a process, and the molar ratio of Ba/Ti is hard to be tailored during the course of washing with water. In addition, the hydrous titanic acid compound prepared via sulfuric acid process inevitably contains residual sulfur compounds in the form of sulfate as impurities, which are hard to be washed off by water and reacts with barium hydroxide during the hydrothermal reaction to form barium sulfate. Barium sulfate is a thermally stable compound being insoluble in water or organic solvent, so that it cannot be removed through water-washing or calcining. Commercially available hydrous titanic acid compound prepared by sulfuric acid process is metatitanic acid, of which the purest one contains from 0.3 to 0.8% by weight of sulfur compounds: if it is employed in preparing barium titanate, the product will contain from about 0.4 to 1.0% by weight of barium sulfate. If the raw material is washed by water to avoid such a result, hydrous titanic acid compound cannot be easily removed due to its ion exchanging ability, with its critical concentration of about 0.3% by weight. The residual barium sulfate in barium titanate essentially lowers the purity of barium titanate as well as the permittivity during the course of manufacturing a multi-layer ceramic condenser, and affects sintering property and temperature property of MLCC. Further, sulfur-containing gas is generated at the sintering stage performed at a temperature of 1300° C. or higher during the process for manufacturing a MLCC to cause corrosion of nickel electrode, thereby resulting in inferiority of a condenser.
The present inventors have exerted all efforts to analyze the problems and seek the solution in order to achieve above-mentioned object of the invention, and as a result of a number of experiments, they found that highly pure particulate barium titanate particles can be obtained with overcoming the problems resulted from conventional hydrothermal synthesis, by efficiently removing the sulfur-containing impurities while employing hydrous titanic acid compound prepared by sulfuric acid process, and also found a method to adjust the molar ratio of Ba/Ti during the course of reaction, to complete the present invention.